Dual Power Sources

I'm trying to create something with some power cells, and I've run into the issue of inconsistent power supply.

I'd like to supplement power from a solar cell with a battery, but I'm not exactly sure how to keep the input to something at a set value. Basically, when the solar cell supply falls below a certain level, I'd like to supply the missing power with the battery.

Anyone have any ideas? I really don't know if any hardware exists for this already. The idea of using a microcontroller came up (and I can write assembly, so using one's not an issue).

Good post, Mheslep, but I can't help thinking that there's an easier approach. The way an automotive electrical system works is that everything runs off of the battery, and the alternator keeps it charged. In this situation, the power cells can be subbed in for the alternator.

Any kind of semiconductor gate will have losses in its on state. Power MOSFETS have the lowest switch losses. I don't know if thats any better than a good Schottky diode (.1, .2V forward bias). In any case, an transistor/FET that's turned on is effectively just a diode.